ETSI ETS 300 915 ed.3 (1998-01)

Digital cellular telecommunications system (Phase 2+) (GSM); Terminal Adaptation Functions (TAF) for services using synchronous bearer capabilities (GSM 07.03 version 5.2.1)

RE/SMG-040703QR2

Digitalni celični telekomunikacijski sistem (faza 2+) – Terminalske prilagajalne funkcije (TAF) za storitve, ki uporabljajo sinhrone nosilne zmogljivosti (GSM 07.03, različica 5.2.1)

General Information

• Status: Published
• Publication Date: 01-Feb-1998
• Technical Committee: SMG 4 - Data Services
• Current Stage: 12 - Completion
• Due Date: 23-Jan-1998
• Completion Date: 02-Feb-1998
• Ref Project: SIST ETS 300 915 E3:2003 - Digital cellular telecommunications system (Phase 2+) (GSM); Terminal Adaptation Functions (TAF) for services using synchronous bearer capabilities (GSM 07.03 version 5.2.1)

Standards Content (Sample)

SLOVENSKI STANDARD
01-december-2003
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Digital cellular telecommunications system (Phase 2+) (GSM); Terminal Adaptation
Functions (TAF) for services using synchronous bearer capabilities (GSM 07.03 version
5.2.1)
Ta slovenski standard je istoveten z: ETS 300 915 Edition 3
ICS:
33.070.50 Globalni sistem za mobilno Global System for Mobile
telekomunikacijo (GSM) Communication (GSM)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN ETS 300 915
TELECOMMUNICATION January 1998
STANDARD Third Edition
Source: SMG Reference: RE/SMG-040703QR2
ICS: 33.020
Key words: Digital cellular telecommunications system, Global System for Mobile communications (GSM)
R
GLOBAL SYSTEM FOR
MOBILE COMMUNICATIONS
Digital cellular telecommunications system (Phase 2+);
Terminal Adaptation Functions (TAF)
for services using synchronous bearer capabilities
(GSM 07.03 version 5.2.1)
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: [e-mail]
Tel.: [phone] - Fax: [phone]
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1998. All rights reserved.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
Contents
Foreword .5
1 Scope .7
2 Normative references.7
2.1 Abbreviations .9
3 General.9
3.1 Customer access configuration .9
3.2 Terminal Adaptation Function.9
3.3 TAF Interfacing to other MT functions .10
4 Terminal Adaptation Functions for synchronous transparent services .10
4.1 Rate Adaptation .10
4.1.1 Rate adaptation - V-series.11
4.1.2 Rate adaptation - X.21 .11
4.1.3 Rate adaptation - S-interface .11
4.2 Interchange Circuit Signalling Mapping .13
4.2.1 V-series interchange circuit mapping .13
4.2.1.1 Multislot configurations (Channel coding TCH/F9.6 or
TCH/F4.8 kbit/s) .15
4.2.1.2 Channel coding TCH/F14.4 .15
4.2.2 X.21 Interchange circuit mapping.15
4.2.3 Case of S-interface.16
4.3 Call establishment signalling mapping at TE/MT interface.17
4.3.1 V-series interfaces.17
4.3.1.1 Call establishment manual operation - utilizing Alternate
Speech/Data or Speech followed by Data Capabilities .17
4.3.1.2 Call establishment manual operation - utilizing the
Unrestricted Digital Capability.17
4.3.1.3 V.25 bis auto call/auto answer.17
4.3.2 X-series interfaces .18
4.3.2.1 X.21 bis call establishment manual operation - utilizing
the Unrestricted Digital Capability.18
4.3.2.2 X.21 bis/V.25 bis call establishment signalling mapping .18
4.3.2.3 X.21 call establishment signalling mapping.18
4.3.3 S-interface (I.420) signalling mapping.18
4.3.4 X.25 procedures mapping .19
5 Terminal Adaptation Functions for synchronous non-transparent services .19
5.1 Rate Adaptation and protocol model.19
5.1.1 R-interface.19
5.1.2 S-interface.19
5.2 Signalling Mapping.19
5.2.1 Interchange circuit signalling mapping .19
5.2.2 Call establishment signalling mapping .19
5.3 Flow Control.19
5.3.1 Conditions requiring flow control towards the network.19
5.3.2 Conditional requiring flow control towards TE2 .20
5.3.3 Local flow control.20
5.4 Buffers .20
5.4.1 TX buffers.20
5.4.2 RX buffers .20
6 V- and S-series interface procedures to 04.08 mapping.21
6.1 Mobile Originated calls.21
6.2 Mobile Terminated calls.22

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
7 X.21 interface procedures to 04.08 mapping. 22
7.1 X.21 procedures mapping. 22
7.1.1 Mobile originated call (see figure 10). 22
7.1.2 Mobile terminated call (see figure 10). 23
7.1.3 Mobile termination clearing (see figure 11). 23
7.1.4 Distant end terminal clearing . 23
7.1.5 Network generated clearing (see figure 11) . 23
7.2 Dm Signalling causes mapping to X.21 call progress signals. 25
7.3. X.21 FACILITIES MAPPING . 25
8 Support for packet service. 27
8.1 Terminal configurations. 27
8.2 Support for basic packet access . 28
8.3 Support for dedicated packet access. 29
Annex A (normative): L2R Functionality . 30
A.1 Introduction. 30
A.2 L2RBOP. 30
A.3 Use of the L2RBOP . 32
A.3.1 Radio Link Connection Control. 32
A.3.2 Status transfer. 33
A.3.3 LAPB connection control. 33
A.3.4 LAPB exchange identification. 33
A.3.5 Data Transfer . 33
A.3.6 Flow control. 33
History. 34

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
Foreword
This European Telecommunications Standard (ETS) has been produced by the Special Mobile Group
(SMG) of the European Telecommunications Standards Institute (ETSI).
This ETS defines the interfaces and Terminal Adaptation Functions (TAF) integral to a Mobile Termination
(MT) which enables the attachment of synchronous terminals to a MT within the digital cellular
telecommunications system (Phase 2+).
The contents of this ETS is subject to continuing work within SMG and may change following formal SMG
approval. Should SMG modify the contents of this ETS, it will be resubmitted for OAP by ETSI with an
identifying change of release date and an increase in version number as follows:
Version 5.x.y
where:
y the third digit is incremented when editorial only changes have been incorporated in the
specification;
x the second digit is incremented for all other types of changes, i.e. technical enhancements,
corrections, updates, etc.
The specification from which this ETS has been derived was originally based on CEPT documentation,
hence the presentation of this ETS may not be entirely in accordance with the ETSI drafting rules.
Transposition dates
Date of adoption of this ETS: 2 January 1998
Date of latest announcement of this ETS (doa): 30 April 1998
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 31 October 1998
Date of withdrawal of any conflicting National Standard (dow): 31 October 1998

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
1 Scope
This European Telecommunications Standard (ETS) defines Terminal Adaptation Functions (TAF) which
are integrated in a Mobile Termination (MT) and which enable the attachment of Synchronous Terminals
to an MT (see GSM 04.02 [4]). The general aspects of Terminal Adaptation Functions are contained in
specification GSM 07.01 (ETS 300 913) [8]. This ETS covers support of synchronous data services (see
GSM 02.02 (ETS 300 904)[2]) for the following interfaces and procedures:
- V.22 DTE/DCE Interface
- V.22 bis DTE/DCE Interface
- V.26 ter DTE/DCE Interface
- V.32 DTE/DCE Interface
- X.21 DTE/DCE Interface
- X.21 bis DTE/DCE Interface
- X.25 Procedure
- X.32 Procedure
- V.25 bis Procedure
- I.420 Interface (S)
LAPB is the only synchronous non-transparent protocol which is considered here.
2 Normative references
This ETS incorporates by dated and undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references, subsequent amendments to or revisions of any of these publications
apply to this ETS only when incorporated in it by amendment or revision. For undated references, the
latest edition of the publication referred to applies.
[1] GSM 01.04 (ETR 350): "Digital cellular telecommunication system (Phase 2+);
Abbreviations and acronyms".
[2] GSM 02.02 (ETS 300 904): "Digital cellular telecommunication system
(Phase 2+); Bearer Services (BS) supported by a GSM Public Land Mobile
Network (PLMN)".
[3] GSM 03.10: "Digital cellular telecommunication system (Phase 2+); GSM Public
Land Mobile Network (PLMN) connection types".
[4] GSM 04.02: "Digital cellular telecommunication system (Phase 2+); GSM Public
Land Mobile Network (PLMN) access reference configuration".
[5] GSM 04.08 (ETS 300 940): "Digital cellular telecommunication system
(Phase 2+); Mobile radio interface layer 3 specification".
[6] GSM 04.21 (ETS 300 945): "Digital cellular telecommunication system; Rate
adaption on the Mobile Station - Base Station System (MS - BSS) interface".
[7] GSM 04.22 (ETS 300 946): "Digital cellular telecommunication system
(Phase 2+); Radio Link Protocol (RLP) for data and telematic services on the
Mobile Station - Base Station System (MS - BSS) interface and the Base Station
System - Mobile-services Switching Centre (BSS - MSC) interface".
[8] GSM 07.01 (ETS 300 913): "Digital cellular telecommunication system
(Phase 2+); General on Terminal Adaptation Functions (TAF) for Mobile
Stations (MS)".
[9] GSM 08.20 : "Digital cellular telecommunication system; Rate adaption on the
Base Station System - Mobile-services Switching Centre (BSS - MSC)
interface".
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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
[10] GSM 09.06 (ETS 300 975): "Digital cellular telecommunication system
(Phase 2+); Interworking between a Public Land Mobile Network (PLMN) and a
Packet Switched Public Data Network/Integrated Services Digital Network
(PSPDN/ISDN) for the support of packet switched data transmission services".
[11] GSM 09.07 (ETS 300 976): "Digital cellular telecommunication system
(Phase 2+); General requirements on interworking between the Public Land
Mobile Network (PLMN) and the Integrated Services Digital Network (ISDN) or
Public Switched Telephone Network (PSTN)".
[12] CCITT Series V Recommendations: "Data communication over the Telephone
network".
[13] CCITT Series X Recommendations: "Data communication networks".
[14] CCITT Recommendation V.10: "Electrical characteristics for unbalanced
double-current interchange circuits for general use with integrated circuit
equipment in the field of data communications".
[15] CCITT Recommendation V.11: "Electrical characteristics for balanced
double-current interchange circuits for general use with integrated circuit
equipment in the field of data communications".
[16] CCITT Recommendation V.25: "Automatic answering equipment and/or parallel
automatic calling equipment on the general switched telephone network
including procedures for disabling of echo control devices for both manually and
automatically established calls".
[17] CCITT Recommendation V.25 bis: "Automatic Calling and/or Answering
Equipment on the General Switched Telephone Network (GSTN) using the
100-series interchange circuits".
[18] CCITT Recommendation V.28: "Electrical characteristics for unbalanced
double-current interchange circuits".
[19] CCITT Recommendation V.110: "Support of data terminal equipments (DTEs)
with V-Series interfaces by an integrated services digital network".
[20] CCITT Recommendation V.24: "List of definitions for interchange circuits
between data terminal equipment (DTE) and data circuit-terminating
equipment".
[21] CCITT Recommendation X.24: "List of definitions for interchange circuits
between Data Terminal Equipment (DTE) and Data Circuit Terminating
Equipment (DCE) on Public Data Networks".
[22] CCITT Recommendation X.25: "Interface between Data Terminal Equipment
(DTE) and Data Circuit Terminating Equipment (DCE) for terminals operating in
Packet Mode and connected to Public Data Networks by dedicated Circuit".
[23] CCITT Recommendation X.21: "Interface between Data Terminal Equipment
(DTE) and Data Circuit Terminating Equipment (DCE) for synchronous
operation in public data networks".
[24] CCITT Recommendation X.21 bis: "Use on public data networks of data
terminal equipment (DTE) which is designed for interfacing to synchronous
V-Series modems".
[25] CCITT Recommendation X.26: "Electrical characteristics for unbalanced
double-current interchange circuits for general use with integrated circuit
equipment in the field of data communications".

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
[26] CCITT Recommendation X.27: "Electrical characteristics for balanced
double-current interchange circuits for general use with integrated circuit
equipment in the field of data communications".
[27] CCITT Recommendation X.30: "Support of X.21, X.21 bis and X.20 bis based
Data Terminal Equipment (DTEs) by an ISDN".
[28] CCITT Recommendation X.31: "Support of Packet Mode Terminal Equipment in
ISDN".
[29] CCITT Recommendation X.32: "Interface between Data Terminal Equipment
(DTE) and Data Circuit Terminating Equipment (DCE) for terminals operating in
Packet Mode and accessing a PSPDN through a PSTN or an ISDN or a
CSPDN".
[30] CCITT Recommendation I.461: "Support of X.21, X.21 bis and X.20 bis based
data terminal equipment (DTEs) by integrated services digital network (ISDN)".
[31] CCITT Recommendation I.463: "Support of data terminal equipment (DTEs)
with V-Series type interfaces by an integrated services digital network (ISDN)".
[32] ISO Recommendation 8885: "Information technology - Telecommunication and
information exchange between systems - High-level data link control (HDLC)
procedures - General purpose XID frame information field content and format".
[33] ISO Recommendation 8886: "Information technology - Telecommunication and
information exchange between systems - Data link service definitions for Open
Systems interconnection".
[34] Personal Computer Memory Card Association: "PCMCIA 2.1 or PC-Card 3.0
electrical specification or later revisions".
[35] Infrared Data Association IrDA "IrPHY Physical layer signalling standard".
[36] TIA-617: "Data Transmission Systems and Equipment - In-Band DCE Control".
[37] GSM 02.34: "Digital cellular telecommunications system (Phase 2+); High
Speed Circuit Switched Data (HSCSD) - Stage 1”
[38] GSM 03.34 (TS 101 038): "Digital cellular telecommunications system
(Phase 2+); High Speed Circuit Switched Data (HSCSD) -Stage 2 Service
Description”
2.1 Abbreviations
In addition to those below abbreviations used in this ETS are listed in GSM 01.04 (ETR 350) [1].
AU Access Unit
PF Packet Function
3 General
3.1 Customer access configuration
The GSM PLMN access reference configuration is described in figure 1 of GSM 04.02 [4]. This
specification (GSM 07.03) specifically refers to the MTs which support terminal equipments (TE1 or TE2)
that use synchronous bearer capabilities.
3.2 Terminal Adaptation Function
The TAF is functionally part of an MT0, MT1 or MT2 (see GSM 04.02 [4]). The terminal adaptation
provides facilities to allow manual or automatic call control functions associated with alternate

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
speech/data, speech followed by data and circuit switched data services, in case of V series interfaces.
The X.21 DTE/DCE interface allows only for automatic call control functions. The following functions are
included:
- Conversion of electrical, mechanical, functional and procedural characteristics of the V-series,
X-series and ISDN type interfaces to those required by a GSM PLMN.
- Bit rate adaptation of V-series and X-series data signalling rates and the ISDN 64 kbit/s to that
provided in the GSM PLMN.
- The mapping of V.25 bis AUTO CALL/AUTO ANSWER procedures and X.21 procedures to the
GSM PLMN Dm-channel signalling.
- The mapping functions necessary to convert S-interface signalling to PLMN Dm-channel signalling.
- Synchronization procedure, which means the task of synchronizing the entry to and the exit from
the data transfer phase between two subscriber terminals. This is described in the specification
GSM 07.01 (ETS 300 913) [8].
- Filtering of channel control information. This is described in the specification GSM 07.01
(ETS 300 913) [8].
- Compatibility checking (see GSM 07.01 (ETS 300 913) [8])
- Layer 2 relaying (see annex 1)
- Flow control
- In Call Modification function (see section 4)
- Splitting and combining of the data flow in case of multislot data configurations
3.3 TAF Interfacing to other MT functions
TAF interfacing is shown in figure 1.
Mobility Management
TAF RR Management
Channel Codec    FEC
Call Control
MMI
Figure 1: TAF interfacing to other MT functions
4 Terminal Adaptation Functions for synchronous transparent services
Specification GSM 03.10 [3] refers to the models for connection types supporting synchronous transparent
services.
4.1 Rate Adaptation
Rate adaptation on the MS-BS interface is described in GSM 04.21. The synchronous data services make
use of the following rate adaptation functions: RA1, RA2, RA1/RA1' and RA1'. See also Figure 6 in GSM
03.10. The D-bits of the rate adaptation frames are used to convey user data and the S- and X-bits are
used to convey channel status information associated with the data bits in the data transfer state, or to
carry substream numbering between the Split/Combine functions in case of multislot operation. For the S-
and X-bits, a ZERO corresponds to the ON condition, a ONE to the OFF condition.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
4.1.1 Rate adaptation - V-series
This is provided as indicated in specification GSM 04.21 (ETS 300 945) [6]. The functions applied in this
case are shown in figure 2 (see model 2b in figure 6 of GSM 03.10 [3]).
Interface circuits
MT2
(data and control)
TE2
V-series
RA1'
R
Figure 2: Rate adaptation for V-series terminals
4.1.2 Rate adaptation - X.21
This is provided as indicated in specification GSM 04.21 (ETS 300 945) [6]. The functions applied in this
case are shown in figure 3 (see model 2b in figure 6 of GSM 03.10 [3]).
Interface circuits
T and R for data
MT2
V and I for control
TE2
X-series RA1'
R
Figure 3: Rate adaptation for X.21 terminals
4.1.3 Rate adaptation - S-interface
The functions applied in this case are shown in figure 4 (see model 2a in figure 6 of GSM 03.10 [3]).
TA MT1
TE2
RA1 RA2 RA1 RA1'
RA2
RS
Figure 4a: Rate adaptation for S-interface
TE1 MT1
RA1 RA2 RA1 RA1'
RA1
S
Figure 4b: Rate adaptation for S-interface (continued)
There are two cases to be considered for the RA1 function:
a) V-series interface
For the V-series type of terminal equipments the rate adaptation functions are as described in
GSM 04.21 (ETS 300 945) [6].
b) X.21-interface
For terminal equipments using the X.21-interface the rate adaptation functions are identical to those
described in GSM 04.21 (ETS 300 945) [6], but the notation used is as described in CCITT
recommendation X.30/I.461.
The notation used is as follows:
The conversion of the user rates of 2.4 and 4.8 kbit/s to 8 kbit/s and user rate of 9.6 kbit/s to
16 kbit/s shall be implemented by means of the 40 bit frame structure shown in figure 5.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
Figure 5 shows that in addition to the basic frame, a two frame multiframe is employed. In odd
frames, octet 0 contains all zeros, whilst in even frames octet 0 consists of a one followed by seven
E bits. The order of bit transmission of the 40 bit frame is from left-to-right and top-to-bottom.
This two frame multiframe corresponds to the 80 bit frame structure presented in GSM 04.21
(ETS 300 945) [6] as shown in figure 6. The 24 information bits P1,.,P8, Q1,.Q8, R1,.,R8 of odd
frames correspond with D1,.,D24 and those of even frames correspond with D25,.,D48
respectively. For the status bits there is the following correspondence: odd frame
SQ,X,SR,SP = S1,X,S3,S4 and even frame SQ,X,SR,SP = S6,X,S8,S9.
Option for a manufacturer of mobile stations:
In transparent mode support of a packet mode TE1 or TE2/TA, which uses flag stuffing.
Bit number
12 34 56 78
Octet 0 Odd frames 0 0 0 0 0 0 0 0
Even frames 1 E1 E2 E3 E4 E5 E6 E7
Octet 1 1 P1 P2 P3 P4 P5 P6 SQ
Octet 2 1 P7 P8 Q1 Q2 Q3 Q4 X
Octet 3 1 Q5 Q6 Q7 Q8 R1 R2 SR
Octet 4 1 R3 R4 R5 R6 R7 R8 SP
NOTE: Bit X, if not used for the optional flow control or for the indication of the far end
synchronization, shall be set to 0 (see CCITT Recommendation I.463/V.110).
Figure 5: 40 bit frame structure of CCITT X.30
X.30 Two frame multifr. V.110 80-bit frame

00000000 00000000
odd 1 P1P2P3P4P5P6SQ 1 D1 D2 D3 D4 D5 D6 S1
frame 1 P7 P8 Q1 Q2 Q3 Q4 X 1 D7 D8 D9 D10 D11 D12 X
1 Q5Q6Q7Q8R1 R2 SR 1 D13 D14 D15 D16 D17 D18 S3
1 R3R4R5R6R7R8SP 1 D19 D20 D21 D22 D23 D24 S4

1 E1 E2 E3 E4 E5 E6 E7 1 E1 E2 E3 E4 E5 E6 E7
even 1 P1 P2 P3 P4 P5 P6 SQ 1 D25 D26 D27 D28 D29 D30 S6
frame 1 P7 P8 Q1 Q2 Q3 Q4 X 1 D31 D32 D33 D34 D35 D36 X
1 Q5Q6Q7Q8R1 R2 SR 1 D37 D38 D39 D40 D41 D42 S8
1 R3R4R5R6R7R8SP 1 D43 D44 D45 D46 D47 D48 S9
Figure 6: Correspondence of X.30 and V.110 frames

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
4.2 Interchange Circuit Signalling Mapping
4.2.1 V-series interchange circuit mapping
The interchange circuit signalling mapping at the interface between the TE2 and the MT shall conform to
CCITT recommendation V.24; while the signal levels at the interface shall conform either to CCITT
recommendation V.28, or to IrDA IrPHY Physical signalling standard specification, or to PCMCIA 2.1, or to
PC-Card 3.0 electrical specifications or to later revisions.
The signals required at this interface are shown in table 2.
Specification 04.21 refers to the frame structure and identifies the use of status bits for the carriage
of signalling information.
Status bits
The bits S and X are used to convey channel status information associated with the data bits in the data
transfer stage as shown below. The S-bits are put into two groups SA and SB to carry the condition of two
interchange circuits. The X-bit is used to control the condition of circuit 106.
The mechanism for proper assignment of the control information from the transmitting signal rate adapter
interface via these bits to the receiving signal rate adapter interface is shown below in table 1.
For the S and X bits, a ZERO corresponds to the ON condition, a ONE to the OFF condition.
General mapping scheme
Table 1: General mapping scheme for V-series interchange circuits
S1, S3, S6, S8 = SA 107
S4, S9 = SB
Frame synch
X
and 106/IWF 106
TE IWF
IWF TE
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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
Table 2: Minimum set of V-series interchange circuits
Circuit Number Circuit Name Ground Data Control
to from to from
TE2 TE2 TE2 TE2
CT102 Common Return X
CT103 Transmitted
data X
CT104 Received data X
CT105 Request to
send X
CT106 Ready for
sending X
CT107 Data set ready X
CT108.2 Data terminal
ready X
CT109 Data channel
received line X
signal detector
CT114 Transmitter
signal element X
timing
CT115 Receiver
signal element X
timing
CT125 Calling in-
dicator (note) X
NOTE: CT125 is used with the AUTO ANSWER function of the TAF.
Use of Network Independent Clocking:
Network Independent Clocking is only applicable to calls using ITC value "3.1 kHz audio ex PLMN".
Within the GSM network the coding of the values for bits associated with NIC is specified in GSM
specifications GSM 04.21 (ETS 300 945) [6]/GSM 08.20 [9]. In the forward (transmitting) direction the
multiframes shall be coded in exact accordance with that specified in those specifications. Bit E6 is set to
"1" in alternate modified V.110 frames at the transmitter. However, the use of this bit at the receiver for
monitoring frame Synchronization, or any other purpose, is not specified and is left to the discretion of the
implementor.
A "perfect linear block Code" is used in C1-C5, whose error correction properties may be utilized in the
receiver, in order to ensure reliable operation of NIC.
The NIC sending function has to recognize when the difference between the applicable clock speed of the
GSM network and the interface speed generates a positive or negative whole bit requirement. When this
positive or negative condition occurs, the NIC codewords specified in specification GSM 04.21
(ETS 300 945) [6] are used to transport this condition to the receiving NIC function. Transmission of the
codeword shall clear the positive or negative condition related to that codeword at the sending function.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
The sending function shall not send more than one positive or negative compensations within a
contiguous period of time corresponding to 10 000 user data bits minus the number of user data bits
necessary to make up an even number of V.110 frames between compensations (NIC compensation is
coded in two V.110 frames). This results from the requirements to compensate for maximum clock
differences of ± 100 parts per million. If the receiving function receives NIC compensations more often
than a contiguous period of time corresponding to 10 000 user data bits, there is no guarantee that data
will not be lost.
The NIC receiving function has to provide the capability to support the compensation requirements of the
sending function. This compensation is managed by manipulating the clock speed of the interface, within
the standard constraints of that interface.
Overall, the compensation functions have to be capable of managing clock tolerances of ± 100 parts per
million.
The NIC function has to recognize and manage the conversion of the NIC information received incoming
from an ISDN terminal Interface. The conversion has to be made to the NIC format used within the GSM
System as defined in specifications 04.21/08.20. The NIC function has to manage the conversion of the
GSM NIC format into that used within the ISDN in the traffic direction towards the ISDN terminal interface.
Due to the incompatibility between the ISDN and the GSM requirements NIC interworking is nor provided
between these two formats. as such no NIC function is required in providing interworking to the ISDN for
unrestricted digital.
Action on loss of synchronization:
If five consecutive NIC multiframes have incorrect framing bit values in E7, the receiver shall stop applying
clocking compensation to the received data. Resynchronization will be attempted and compensation will
resume when synchronization is achieved.
Signal element timing:
Receiver signal element timing (CT115) is generated by MT2. In the transparent case, this shall be
synchronized to the output of RA1' function. In the non transparent case it is output from the L2R on the
basis of the current user data rate. A transition from ON to OFF condition shall nominally indicate the
centre of each signal element on CT104.
Transmitter signal element timing is generated by MT2 (CT114), this may be synchronized to CT115.
In the case of alternate Speech/Group 3 Facsimile, there may be a Channel Mode Modify during the
course of the facsimile portion of the call. If this occurs, the user data rate changes and this is reflected to
the V.24 interface as a change in the clock speed on CT 114 and CT 115.
4.2.1.1 Multislot configurations (Channel coding TCH/F9.6 or TCH/F4.8 kbit/s)
In transparent multislot configurations status bits S1, S3 and the X-bit between the D12 and D13 in the
ITU-T V.110 80-bit intermediate rate frame - are used for transferring substream numbering information.
The S4-bit is used for frame synchronization between the parallel substreams (ref GSM 04.21).
4.2.1.2 Channel coding TCH/F14.4
For information on the mapping of the interchange circuit signalling bits in the 14.5 multiframe structure,
refer to GSM 04.21.
4.2.2 X.21 Interchange circuit mapping
The interchange circuit signalling mapping at the interface between the TE2 and the MT shall conform to
CCITT recommendations X.21 and X.24; while the signal levels at the interface shall conform either to
CCITT recommendation X.26 (v.10), or to X.27 (V.11) - see also paragraph 2.1 of CCITT
recommendation X.21, or to IrDA IrPHY Physical signalling standard specification, or to PCMCIA 2.1, or to
PC-Card 3.0 electrical specifications or to later revisions.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
The signals required at this interface are shown in table 3.
Specification 04.21 refers to the frame structure and identifies the use of status bits for the carriage of
signalling information.
Status bits (S1,S3,S4,S6,S8,S9):
For the purpose of alignment with the case where the X.21 TE2 is connected to the MT via a TA
conforming to CCITT recommendation X.30 (I.461), the notation for the S-bits will be SP, SQ and SR as in
figure 5/GSM 07.03. For the bits SP, SQ and SR, a ZERO corresponds to the ON condition, a ONE to the
OFF condition.
The bits SP, SQ and SR are used to convey channel associated status information. The mapping of the
information on circuit C of the X.21 interface to the S bits and from the S bits to the circuit I in the distant
interface should be done in such a way that the SP, SQ and SR bits are associated with the bit-groups P,
Q and R. To assure proper and secure operation the mapping scheme has to be consistent with CCITT
recommendations X.21 and X.24.
The mechanism for mapping is as follows:
- In all cases where X.21-byte timing interchange circuit B is not provided, the status bits SP, SQ and
SR of the bit groups P, Q and R are evaluated by sampling the circuit C in the middle of the 8th bit
of the respective preceding bit group. On the other hand, the conditions of the status bits SP, SQ
and SR are adopted by the circuit I beginning with transition of the respective 8th bit of a bit-group
P, Q and R to the first bit of the consecutive bit group on the circuit R.
- In the case where X.21-byte timing interchange circuit B is provided for character alignment, the
circuit C is sampled together with the bit 8 of the preceding octet and the circuit I is changing its
state at the boundaries between the old and new octets at the circuit R. This operation is defined in
CCITT recommendation X.24.
Table 3: X.21 interchange circuits
Interchange Interchange circuit Data Control Timing
circuit name toTE2
to from to from
TE2 TE2 TE2 TE2
G Common return
Ga TE2 common return
T Transmit X X
R Receive X
C Control X
I Indication X
S Signal element timing X
B Byte timing (note) X
NOTE: According to CCITT recommendation X.21 the provision of the 8 bit timing interchange
circuit B is not mandatory.
4.2.3 Case of S-interface
At the S-interface an X.30 rate adapted bit stream is provided by the TE1 or TE2-TA combination (see
figure 4). The terminal adaptation function within the MT does not have any interchange circuit signalling
mapping function to perform.
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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
4.3 Call establishment signalling mapping at TE/MT interface
4.3.1 V-series interfaces
4.3.1.1 Call establishment manual operation - utilizing Alternate Speech/Data or Speech
followed by Data Capabilities
During manual call establishment, the mobile user shall be able to hear network supervisory tones and
answer tone.
On hearing answer tone, the user invokes the transition from speech to data in both Mobile Station and
the IWF. The mapping for this is shown in section 6.
4.3.1.2 Call establishment manual operation - utilizing the Unrestricted Digital Capability
In this case the user will not hear network supervisory tones or answer tone. The data transfer phase will
be entered automatically.
4.3.1.3 V.25 bis auto call/auto answer
The mapping of the V.25 bis procedures to the messages of the PLMN Dm-channel signalling
(GSM 04.08 (ETS 300 940) [5]) is defined in section 4.
Auto Call:
This procedure is provided according to V.25 bis using only circuit 108/2. A subset of V.25 bis is shown in
table 4. This subset gives minimum level of control and indication.
During the call establishment phase, i.e. after signalling, call tone according to V.25 bis shall be generated
in the IWF, where appropriate.
Auto Answer:
This procedure is provided according to V.25 bis.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
Table 4: Minimum set of V.25 bis Call Set-up Commands and Indications
Description IA5Characters
Commands Call Request with Number CRN
from TE2 provided 0,1.9,*,#,A,B,C,D
Disregard Incoming Call DIC
Connect Incoming Call CIC
Indications Call Failure Indication CFI XX
to TE2 XX = CB,AB,NT,FC (Note)
INcoming Call INC
VALid VAL
INValid INV
NOTE to table 4: CB = Local MT busy
AB = Abort call
NT = No answer
FC = Forbidden call *
* Forbidden call indication results from contravention of rules for repeat call attempts as
defined by the appropriate national approvals administration. It is recommended that
this is the responsibility of the MT, not the TE2.
4.3.2 X-series interfaces
4.3.2.1 X.21 bis call establishment manual operation - utilizing the Unrestricted Digital
Capability
In this case the user will not hear network supervisory tones or answer tone. The data transfer phase will
be entered automatically.
4.3.2.2 X.21 bis/V.25 bis call establishment signalling mapping
The mapping of the V.25 bis procedures to the messages of the PLMN Dm-channel signalling
(GSM 04.08 (ETS 300 940) [5]) is defined in section 6.
Auto Call:
This procedure is provided according to V.25 bis using only circuit 108/2. A subset of V.25 bis is shown in
table 4. This subset gives minimum level of control and indication.
Auto Answer:
This procedure is provided according to V.25 bis.
4.3.2.3 X.21 call establishment signalling mapping
The mapping of the X.21 procedures to the messages of the PLMN Dm-channel signalling (GSM 04.08
(ETS 300 940) [5]) is defined in section 7.
4.3.3 S-interface (I.420) signalling mapping
The mapping of Q.931 signalling to 04.08 signalling requires the inclusion, by the MT, of PLMN specific
elements (eg. transparent or not, half or full rate channel). The required Bearer Capability Elements are
shown in GSM 07.01 (ETS 300 913) [8] Annex 2.

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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
4.3.4 X.25 procedures mapping
User terminals are connected to mobile termination either at S reference point (TE1 or TE2/TA) or at R
reference point (TE2). For the physical interface of TE2s all different possibilities are shown in table 9 in
section 8.
For more details, see CCITT X.25 and the appropriate interface recommendations.
The mapping is described in section 8.
5 Terminal Adaptation Functions for synchronous non-transparent services
This section deals with the specific requirements for non-transparent X.25 access. Other cases, e.g.
teletex, are dealt within other specifications.
Layer 2 Relay function is described in annex 1.
5.1 Rate Adaptation and protocol model
5.1.1 R-interface
For the protocol model and rate adaptation function applied in this case see Models 4b and 4e of Figure
6/GSM 03.10).
5.1.2 S-interface
For the cases where the method indicated in CCITT X.30 is used see Models 4a and 4d of Figure
6/GSM 03.10).
For the cases where the HDLC interframe flag stuffing shown in the recommendation CCITT X.31 is used
see Models 4c and 4f of Figure 6/GSM 03.10).
5.2 Signalling Mapping
5.2.1 Interchange circuit signalling mapping
The interchange circuit signalling mapping is identical to the transparent case described in section 4.2.
5.2.2 Call establishment signalling mapping
The physical interfaces are mentioned in section 4.3.4 and the signalling mapping is described in
section 8.
5.3 Flow Control
The passage of flow control information between L2Rs is described in annex 1.
5.3.1 Conditions requiring flow control towards the network
The L2R function will send immediately a "flow control active" indication in the following circumstances:
(i) If the receive buffer from the radio side reaches a preset threshold.
(ii) If local flow control is initiated by the TE2 (see section 5.3.3 a)). On receipt of this flow control
indication transmission of data from the receive buffer towards the TE2 is halted.
On removal of the buffer congestion or local flow control the L2R will send a "flow control inactive"
indication.
In addition, for the local flow control condition, transmission of data from the receive buffers will be
restarted.
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ETS 300 915 (GSM 07.03 version 5.2.1): January 1998
5.3.2 Conditional requiring flow control towards TE2
The L2R function will immediately activate local flow control (see section 5.3.3 b)) under the following
circumstances:
(i) The transmit buffer reaches a pre-set threshold.
(ii) The L2R receives a "flow control active" indication.
On removal of the buffer congestion or receipt of L2R/RLP "flow control inactive" the local flow control will
be removed.
5.3.3 Local flow control
Only inband flow control is allowed:
a) from TE2:
RNR is sent to indicate flow control active. RR is sent to indicate flow control inactive. Where
RR/RNR is utilized then the TAF will generate flow control active/inactive immediately.
b) From TAF: As from TE2.
Where this method is used, the L2R will pass the RNR/RR frames to the TE2.
5.4 Buffers
5.4.1 TX buffers
Data received from the TE2 shall be buffered such that if the MT is unable to transfer the data over the
radio path then data is not lost.
The buffer
...